1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to mobility procedures in communication systems.
2. Description of the Related Art
Conventional communication systems use a network of access nodes to provide network connectivity to one or more access terminals using wired and/or wireless connections. Network architectures generally fall into two broad categories: hierarchical and distributed. Hierarchical network architectures used centralized entities to handle mobility management and radio resource control. For example, in conventional hierarchical communications, a server transmits voice and/or data signaling destined for a target access terminal to a central element such as such as a Radio Network Controller (RNC). The RNC may then transmit paging messages to the target access terminal via one or more access nodes to locate the target access terminal. The target access terminal may establish a communication link to one or more of the access nodes in response to receiving the page from the network. A radio resource management function within the RNC receives the voice and/or data signaling and coordinates the radio and time resources used by the set of access nodes to transmit the information to the target access terminal. The radio resource management function can perform fine grain control to allocate and release resources for broadcast transmission over a set of access nodes.
In contrast, a distributed network includes access points that implement distributed communication network functionality. For example, each distributed access point may combine part or all of the RNC and/or Packet Data Serving Node (PDSN) functions in a single entity that manages radio links between one or more access terminals and an outside network, such as the Internet. Distributed access points may implement proxy functionality that utilizes core network element support to equivalent IP functions. For example, IP anchoring in a UMTS base station router may be offered through a Mobile IP Home Agent (HA) and the Gateway GPRS Support Node (GGSN) anchoring functions that the base station router proxies through equivalent Mobile IP signaling. Compared to hierarchical networks, distributed architectures have the potential to reduce the cost and/or complexity of deploying the network, as well as the cost and/or complexity of adding additional access points to expand the coverage of an existing network. Distributed networks may also reduce (relative to hierarchical networks) the delays experienced by users because packet queuing delays at the RNC and PDSN of hierarchical networks may be reduced or removed.
At least in part because of these advantages, many proposed standards and/or protocols for the WiMAX communication system assume that the WiMAX system can implement a distributed architecture of access nodes. Consequently, many of the mobility related procedures set forth in the proposed standards are designed to support and/or enhance distributed functionality. For example, access service networks in a WiMAX network implement functional entities such as a paging controller, an anchor authenticator, and an anchor data path function to support communication links with each access terminal. The functional entities are independently migrated between access service networks belonging to the same operator as the access terminal moves through the WiMAX network. However, the optimal criteria and/or conditions for migration of the different entities may be different for each entity. The proposed standards take advantage of the flexibility of the distributed network to accommodate these differences by specifying a different procedure for migration of each entity between access service networks belonging to the same operator. The multiple independent migration procedures may, however, represent significant additional overhead, particularly in hierarchical systems.